1. Field of the Invention
This invention relates to a thrust ball bearing for use with power rollers. More particularly, the invention relates to a thrust ball bearing utilized for supporting thrust loads exerted on each of the power rollers, which for example, form part, of a toroidal-type continuously variable transmission.
2. Related Background Art
The use of a toroidal-type continuously variable transmission, which is schematically shown in FIG. 3 and FIG. 4, is under study as a speed change gear for an autovehicle or various kinds of industrial machinery. This toroidal-type continuously variable transmission is such that a disc 2 on the input side is supported coaxially with an input shaft 1, and a disc 4 on the output side is fixed to the end portion of an output shaft 3 which is coaxially arranged with the input shaft 1 as disclosed in the specification of Japanese Utility Model Laid-Open Application No. 62-71465, for example. On the inner side of the casing where the toroidal-type continuously variable transmission is housed or in the supporting brackets arranged in this casing, trunnions 6 and 6 are provided swingably centering on the pivots 5 and 5 arranged in orthogonal positions with respect to the input shaft. 1 and output shaft 3.
The trunnions 6 and 6 are provided with the pivots 5 and 5 on the outer faces of both ends. On the central portions of the trunnions 6 and 6, the base ends of the displacement shafts 7 and 7 are supported, respectively. The trunnions 6 and 6 are allowed to swing centering on the pivots 5 and 5 to make it possible to freely adjust each inclined angle of the displacement shafts 7 and 7. Around the displacement shafts 7 and 7 supported by the trunnions 6 and 6, power rollers 8 and 8 are rotatively supported, respectively. Each of the power rollers 8 and 8 is pinched between the discs 2 and 4 on the input and output sides.
The inner faces 2a and 4a, where the discs 2 and 4 on the input and output sides face each other, are concave in their respective cross-sections, each of which is obtainable by rotating its circle centering on the aforesaid pivot 5. The spherically convex circumferential surfaces 8a and 8a of the power rollers 8 and 8 abut the inner faces 2a and 4a.
Between the input shaft 1 and the disc 2 on the input side, a pressing device 9 of a loading cam type is arranged. By this pressing device 9, the disc 2 on the input side is pressed elastically toward the disc 4 on the output side. The pressing device 9 includes a cam plate 10 which rotates together with the input shaft 1, and a plurality of rollers 12 (four rollers, for example) supported by a cage 11. On one side face (on the left side face in FIG. 3 and FIG. 4) of the cam plate 10, a concave and convex cam surface 13 is formed. Likewise, on the outer surface (on the right side face in FIG. 3 and FIG. 4) of the disc 2 on the input side, a cam surface 14 is formed. Thus the plurality of rollers 12 are rotatively supported in the radial direction around the axis with respect to the center of the input shaft 1.
When the toroidal-type continuously variable transmission structured as described above is used, the plurality of rollers 12 are pressed against the cam surface 14 on the outer side face of the disc 2 on the input side by the rotation of the cam surface 13 and the cam plate 10 following the rotation of the input shaft 1. Therefore, at the same time that the disc 2 on the input side is pressed against the power rollers 8 and 8, the disc 2 on the input side rotates when the pair of cam surfaces 13 and 14 engage with the plurality of rollers 12. The rotation of the disc 2 on the input side is transmitted to the disc 4 on the output side through the power rollers 8 and 8. In this way, the output shaft 3 fixed to the disc 4 on the output side rotates.
When the rotational speed of the input shaft 1 and output shaft 3 should be changed, and speed reduction should be performed the input shaft 1 and output shaft 3, the trunnions 6 and 6 are caused to swing around the pivots 5 and 5 to incline the displacement shafts 7 and 7, respectively, as shown in FIG. 3. The circumferential surfaces 8a and 8a of the power rollers 8 and 8 can abut the portion close to the center of the inner side face 2a of the disc 2 on the input side, and the portion close to the outer circumference of the inner side face 4a of the disc 4 on the output side.
In order to accelerate, the trunnions 6 and 6 are caused to swing to incline the displacement shafts 7 and 7, respectively, as shown in FIG. 4. The circumferential surfaces 8a and 8a of the power rollers 8 and 8 can abut the portion close to the outer circumference of the inner side face 2a of the disc 2 on the input side, and the portion close to the center of the inner side face 4a of the disc 4 on the output side, If each of the displacement shafts 7 and 7 is set at an intermediately inclined angle between those shown in FIG. 3 and FIG. 4, an intermediate reduction ratio can be obtained between the input shaft 1 and output shaft 3.
In this respect, FIG. 3 and FIG. 4 are views which show only the fundamental structure of a toroidal-type continuously variable transmission, but there have hitherto been known various structures which are more specific as transmissions for an autovehicle and others, such as disclosed on the microfilm of Japanese Utility Model Application No. 61-87523 (Japanese Utility Model Laid-Open Application No. 62-199557).
Now, when a toroidal-type continuously variable transmission described above is in operation, the power rollers 8 and 8 rotate at a high speed while receiving the thrust loads from the disc 2 on the input side and the disc 4 on the output side. Therefore, as shown in FIG. 5, a thrust ball bearing 15 is arranged between each power roller 8 and the corresponding trunnion 6.
The thrust ball bearing 15 comprises the power roller 8 which functions as an inner ring; a plurality of balls 16 a cage 20 for rotatively holding the balls 16; and an outer ring 17. The power roller 8, balls 16, and outer ring 17 are formed from bearing steel or ceramic. Also, on the one axial side face (upper face in FIG. 5) of the power roller 8, the inner ring raceway 18 is formed. On the other axial side face (lower face in FIG. 5) of the outer ring 17, where it faces the inner ring raceway 18, the outer ring raceway 19 is formed. These raceways 18 and 19 are sectionally circular, but annular as a whole. The rolling surfaces of the balls 16 are rotatively in contact with the inner and outer ring raceways 18 and 19.
Also, the cage 20 has an annular main body 21 made of metal or synthetic resin. On the main body 21, circular pockets 22 are formed in positions at equal intervals in the circumferential direction between the inner and outer diameters. Each of the balls 16 is rotatively held in each of the pockets 22 in one to one correspondence. Further, the outer ring 17 is abutted upon the inner side face of each trunnion 6 through a thrust bearing 23, such as a needle bearing or a sliding bearing. The thrust bearing 23 allows the trunnion 6 to be displaced with respect to the outer ring 17. The displacement shaft 7 supports the base portion rotatively on the trunnion 6 in order to enable the power roller 8 to follow any deviation of the discs 2 and 4 on the input and output sides, and the leading end of the displacement shaft 7 is arranged eccentrically with respect to the base portion. On this leading end, the power roller 8 is rotatively supported. Therefore, when the power roller follows the deviation of the discs, the outer ring 17 is allowed to be displaced with respect to the trunnion 6. Therefore, the thrust bearing 23 is provided as described above in order to reduce the force required for this displacement, and enhance the capability of the power roller 8 to follow its counterpart.
When the toroidal-type continuously variable transmission is in operation, the thrust ball bearing 15 rotates at a high speed while supporting the thrust loads exerted on each of the power rollers 8 Therefore, the contact angle (an angle formed by the direction of loads given from the balls 16 to each of the raceways, and the plane perpendicular to the central axis of the thrust bearing 15), which is conventionally considered for the thrust ball bearing 15 used in a toroidal-type continuously variable transmission, has been 90 degrees, as represented in FIG. 6. A reference numeral 24 in FIG. 5 designates lubricators for supplying lubricant to the thrust ball bearing 15.
When thrust ball bearings are actually incorporated in a toroidal-type continuously variable transmission for supporting the power rollers 8, there are the points (1) and (2) described below which are yet to be solved.
(1) The distance is not necessarily small in the radial direction between the acting point of loads exerted on the power roller 8 by the disc 2 on the input side and the disc 4 on the output side, and the contacting point of the rolling surface of each of the balls 16 and the inner ring raceway 18. As a result, depending on the loads exerted on each power roller 8 from the aforesaid acting point, great bending stresses tend to be applied to the power roller 8. In other words, when the toroidal-type continuously variable transmission is in operation, loads F.sub.0 are applied locally to the circumferential surface 8a of the power roller 8 by the inner side faces 2a, 4a of the discs 2, 4 in the direction perpendicular to this circumferential surface 8a. Then, in accordance with the component force F.sub.1 of the loads F.sub.0 in the thrusting direction, bending stresses are exerted on the power roller 8 centering on the aforesaid contacting point.
The magnitude of bending stresses is proportional to the distance L.sub.0 between the aforesaid acting point and contacting point in the radial direction. In the conventional toroidal-type continuously variable transmission, this distance L.sub.0 is not necessarily small, and the aforesaid bending stresses become comparatively large. Hence there is a possibility that damage such as cracking occurs in the power roller when it is used for a long time.
(2) When loads are applied to the power roller 8 in the radial direction while a toroidal-type continuously variable transmission is in operation, the power roller 8 tends to be displaced in the radial direction. In other words, on the circumferential surface of the power roller 8, loads are exerted not only in the thrusting direction, but also, exerted in the radial direction in accordance with the displacement of the displacement shaft 7 resulting from the changes of speed or in accordance with its assembling precision and dead weight. Thus in accordance with the radial loads, the power roller 8 is displaced in the radial direction. Particularly, if the contact angle of each of the balls 16 is 90 degrees as in the conventional case, the blocking forces are weak when the power roller 8 tends to be displaced by the application of the radial loads. This as shown in FIG. 7, the power roller 8 is eventually displaced in the radial direction with respect to the outer ring 17.
As a result, the contact angle .theta. of each of the balls 16 becomes more than 90 degrees in certain positions (on the right side in FIG. 7) in the circumferential direction while it becomes less than 90 degrees in some other positions (on the left side in FIG. 7). If the contact angle .theta. varies about 90 degrees in such a manner as described above, the ball undergoes an extremely complicated motion in addition to the motion usually observed. In other words, it revolves while rotating on its own axis because the contact angle .theta. varies continuously. Then, in addition to such a complicated motion as this, influence is exerted by the gyromoment and spin of each ball to produce adverse effects on the balls 16, inner ring raceway 18, and outer ring raceway 19, thereby allowing them to be subjected to frictional wear and shortened fatigue life. Thus the durability of the thrust ball bearing is inevitably diminished. In consideration of these circumstances, the present invention is designed to provide a thrust ball bearing for use with power rollers.
The structure and operation of a toroidal-type continuously variable transmission shown in FIG. 10 are substantially the same as those described in conjunction with FIG. 5. Therefore, the description thereof will be omitted.
When a toroidal-type continuously variable transmission is in operation, the thrust ball bearing 15 rotates at a high speed while supporting the thrust loads exerted on each of the power rollers 8. At this juncture, lubricant is supplied to the thrust ball bearing 15 through the lubricator 24. As shown in FIG. 11, the dimension t of the thickness of the outer ring 17, which forms the thrust ball bearing 15 serving such a purpose as described above, is conventionally formed evenly from the inner circumferential edge to the outer circumferential edge (with the exception of the portion of outer ring raceway 19). In other words, the one axial side face 25 of the outer ring 17 which constitutes the outer ring raceway 19, lies in a single plane which intersects the axis with the exception of the outer ring raceway 19. Also, the one axial side face 26 of the power roller 8 which functions as the inner ring, lies in a single plane which intersects the axis with the exception of the inner ring raceway 18. Further, both side faces of the cage 20 lies in a single plane which is parallel to each of the axial side faces 25 and 26.
However, when the thrust ball bearing 15 is actually incorporated in a toroidal-type continuously variable transmission to support each power roller 8, it is difficult for the reasons given below to attempt making the bearing smaller while securing sufficient durability.
The width dimension W (FIG. 10) of the trunnion 6 for supporting the power roller 8 through the aforesaid thrust ball bearing 15 cannot be made too large, since it is desirable to keep the construction lightweight and to avoid interference between both side edges of the trunnion 6 and the inner side faces 2a, 4a of the discs 2, 4 on the input and output sides. As a result, the outer diametral dimension D of the outer ring 17 of the thrust ball bearing 15 becomes larger than the aforesaid width dimension W. Thus a part of the portion close to the outer circumference of the outer ring 17 is projected from both end edges of the trunnion 6. This part of the portion close to the outer circumference is of course projected outward in the diametral direction from the thrust ball bearing 23 arranged between the trunnion 6 and the outer ring 17.
Particularly, when a part of the outer ring raceway 19 is projected outwardly beyond the thrust bearing 23 in the diametral direction, great bending stresses are exerted on the outer ring 17 due to the thrust loads applied from the power roller 8 to the outer ring 17 through the balls 16. In order to secure the durability of the outer ring 17 despite such bending stresses, the thickness dimension t of the outer ring 17 might be made larger.
However, if the thickness dimension t of the outer ring 17 were made larger, the height dimension H would become larger for the thrust ball bearing for use with power rollers including this outer ring 17 and power roller 8. It is not desirable to increase this height dimension H because such increase directly results in a larger size of the toroidal-type continuously variable transmission and increased weight as well. Also, if the aforesaid thickness dimension t is increased while maintaining the height dimension H at a constant level, the thickness dimension of the power roller 8 would have to be made smaller to that extent, resulting inevitably in a narrower width of the circumferential surface 8a of the power roller 8. The circumferential surface 8a functions as the so-called traction surface which transmits the power between the aforesaid inner side faces 2a and 4a. It is not desirable, to reduce the width dimension of a circumferential surface such as this because it results directly in the lowered efficiency of the power transmission. Therefore, in consideration of these circumstances, the present invention is designed to provide a thrust ball bearing for use with power rollers.